Automatic battery-maintaining implantable cardioverter defibrillator and method for use

ABSTRACT

A device and method for performing automatic battery maintenance as particularly applied in an implantable cardioverter defibrillator (ICD). The battery is maintained at a predetermined state-of-charge to enable charging a capacitor in the ICD to discharge high voltage pulse into a human patient via electrodes implanted in the patient.

This is a continuation of application Ser. No. 08/549,284 filed on Oct.27, 1995 now U.S. Pat. No. 5,690,685.

FIELD OF THE INVENTION

The present invention deals with a battery powered implantablecardioverter defibrillator (ICD) in which the battery is maintained at apredetermined level of voltage and current capacity to reliably supplyenergy to charge the capacitors of the ICD on demand over an extendedlife of the device.

DESCRIPTION OF THE PRIOR ART

ICD's pose a unique demand on the battery power supply. The battery mustbe capable of operating at low current drains for long periods of time,and simultaneously be capable of providing high current pulses. Thepower supply must sustain the provision of the high current pulses andmust be available whenever a patient requires defibrillation.

Batteries which are used in current ICD's can develop high internalimpedance or equivalent series resistance (ESR) over the lifetime of theICD. This is undesirable because a battery with an excessively high ESRwill ultimately be unable to deliver sufficient current to the circuitryof the ICD leading to a potentially catastrophic result to the patient.Batteries with an ESR that is higher than normal also take longer tocharge a capacitor of the ICD, thus potentially delaying the delivery ofvital therapy to the patient. Under normal operations a battery in ICDshould be capable of charging a capacitor in less than about fifteenseconds. Batteries with a higher than normal ESR may take five times aslong.

Accordingly it is imperative to limit the development of a high ESR inICD batteries. Generally, when a battery reforms a capacitor on aperiodic basis, the battery system itself goes through a recharge cycleand this process usually eliminates some ESR that has built up in thebattery system. While this technique is effective in reducing batteryESR, it can not be used more often than a monthly capacitor reformingcycle to routinely limit ESR development in the battery because theprocess is extremely energy-inefficient. For example, each reformingcharge of the capacitor may require the withdrawal of 30-40 joules ofenergy from the battery. If this is performed on a regular schedule soas to limit ESR development in the battery, it would squander a largeamount of battery energy.

While existing ICD batteries have proven effective, it would bedesirable to improve the effectiveness of ICD batteries by limiting ESRdevelopment in ICD batteries such that the batteries are maintained at apredetermined ideal state of voltage and current delivery capacity.

SUMMARY OF THE INVENTION

The present invention maintains a battery system of an implantablecardioverter defibrillator (ICD) system by periodically measuring astate-of-charge of the battery system, and controlling the charging of acapacitor system within the ICD from the battery system based upon themeasured state-of-charge value. Preferably, the state-of-charge value isan electrical parameter value, such as a voltage value or current value.Alternatively, the state-of-charge value may be based on a chargingfrequency at which the battery system charges the capacitor systemthrough a switched high voltage transformer.

In a preferred embodiment, if the state-of-charge value falls below apredetermined value, a battery loading maintenance cycle is activated.Once a battery system reaches a predetermined state-of-charge value, theloading cycle is terminated. In one embodiment, the initiation of acheck of the state-of-charge value is metered by determining a giventime period since the last full charge of the capacitor system has beenperformed.

In another alternate embodiment, the present invention detects a highESR in the battery system during the charging of the capacitor systemand, in response, modifies the charging frequencies so as to reduce theload on the battery system for a period of time sufficient for thebattery system to resolve the high ESR.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a typical implantablecardioverter defibrillator (ICD) circuit.

FIG. 2 is a conceptual illustration of SVO cell design, with cathodeplates connected in parallel in another plane (not shown).

FIG. 3 is a plot of cell voltage vs. time during two simulated chargingcycles illustrating a case of severe voltage delay indicative of a highESR condition.

FIG. 4 is a graphical representation of battery voltage versus deliveredcharge for both an unloaded and a loaded circuit.

FIG. 5 is a schematic representation of an implantable cardioverterdefibrillator (ICD) circuit of the present invention.

FIG. 6 provides a timing diagram for the functioning of an electroniccircuit in accord with the practice of the present invention.

FIG. 7 illustrates a portion of the invertor circuitry of a deviceembodying the present invention.

FIG. 8 is an illustration of a digital control circuit complementary tothat shown in FIG. 7.

FIG. 9 illustrates a portion of an analog circuit capable of functioningin accord with the present invention.

FIG. 10 is an illustration, as represented by a flow chart, of anembodiment of the present invention.

FIG. 11 is an illustration, as represented by a flow chart, of analternative embodiment of the present invention.

FIG. 12 is an illustration, as represented by a flow chart, of analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A typical circuit for an ICD utilizing a silver vanadium oxide (SVO)battery is illustrated in FIG. 1, in which the circuitry is showngenerally at 10. Battery 13 delivers a current through the primary 21 oftransformer 15 upon the actuation of transistor 18. When the transistor18 is turned off (as it is done repeatedly in typical operation), avoltage is generated across the secondary 25 of the transformer, whichvoltage is then directed through diode 27 to charge capacitor 30. Whenthe capacitor 30 is charged to the required voltage, switch 32 is turnedon, and the charge of the capacitor 30 is delivered to cardiacelectrodes through electrodes 35.

The most common type of energy storage means in current ICD's arelithium/silver vanadium oxide (SVO) batteries. In such a battery design,the active cathode material is silver vanadium oxide. Silver vanadiumoxide belongs to a class of nonstoichiometric compounds known asvanadium oxide bronzes. These compounds are semiconductors and exhibittunnel-like crystal structures which provide diffusion paths for metalions. It has been demonstrated that the optimum composition of thecathode material is Ag₂ V₄ O₁₁. Cell discharge takes place in multiplesteps. The first two steps, which occur simultaneously, are thereduction of vanadium (V) to vanadium (IV) and the reduction of silver(I) to silver (0). The final step is the reduction of vanadium (IV) tovanadium (III). Because of the reduction of silver to the metallicstate, the conductivity of the cathode increases during discharge.Stoichiometrically, one mole of Ag₂ V₄ O₁₁ can react with a total ofseven moles of lithium.

The anode in an SVO cell is pure lithium metal pressed onto a nickelcurrent collector. The cathode material is a mixture of silver vanadiumoxide, a Teflon binder, and a conductive carbon material. Individualcathode plates are formed by pressing the cathode material onto a metalcurrent collector to produce a structurally sound pellet. The pelletcontains 95% live cathode material. The anode 31 is folded into an"accordion" design, and individual cathode plates 39 are placed betweeneach "fold" of the anode, as illustrated conceptually in FIG. 2. Boththe anode and the cathode plates are enclosed in an organic separatorfor redundancy.

The chemical reactions occurring in the cell result in a constantlychanging chemical composition of the cathode as the cell reactionproceeds, with a resultant change in the energetics of the reaction.This phenomenon leads to a discharge curve which exhibits plateaus atvarious voltages and a general gradual decline in the voltage as normalbattery discharge proceeds over extended periods of time. The presentinvention recognizes that it is possible to assess the state ofdischarge of the cell by interrogating the cell voltage duringdischarge.

In FIG. 3, a plot of voltage vs. time for an ICD with circuitry of thetype in FIG. 1 is shown. The battery 13 was left unattended for severalmonths. At the start of charging, the cell had an open circuit voltageof 2.51 V which decreased to 1.35 V. During the charging, the voltagegradually recovered to 1.91 V. After a 15 s rest period, anothercharging cycle was initiated which had an initial voltage of 2.00 V.Thus, the battery 13 had recovered to normal operation for the secondshock charging cycle. However, the charging performance during the firstcycle was well below expected performance characteristics for an ICD. Ina clinical setting, such a delay in application of a countershock fromthe ICD's capacitors could have potentially disastrous consequences forthe patient in whom the device is implanted. The data of FIG. 3illustrate the effects of failure to maintain an SVO battery and theconsequent development of an equivalent series resistance (ESR)condition in the battery. The observed phenomenon during the initialcharging cycle can be prevented by periodic capacitor charging,currently necessary for capacitor "reforming." However, as the designand fabrication of capacitors continues to improve, the requirement forperiodic capacitor "reforming" will eventually become unnecessary, andthere may be no other need for this regular and highly inefficient useof stored battery energy.

FIG. 4 illustrates the voltage change with discharge of the cell. Itshows a typical discharge curve 37 for an SVO cell of nominal dimensionsof 43×27×9 mm and a volume of 10.3 cm³. Curve 37 shows the results of aone year test in which the cell was subjected to a constant backgroundload of 17.4 KΩ. (The 17.4 KΩ is a more severe load than the typical ICDmonitoring circuitry provides; the one year is correspondingly muchshorter than the typical ICD lifetime.) Simulating charge cycles for 4defibrillation shocks every two months, the cell is required to producea train of 4 pulses of 2 A each. Each pulse is of 10 s duration with a15 s pause between pulses to accurately model the charge and retentiontimes.

Curve 37 shows the measured voltage under the 17.4 KΩ background(monitoring) load. This quantity is referred to as the open circuitvoltage as it measures the voltage while the cell is under a small ornonexistent load. (The monitoring current is very small compared to theload during capacitor charging.) Curve 37 demonstrates that the originalopen circuit voltage (≈3.2 volts) is level for a first period of time,then decreases for a second period of charge delivery, and then has avery flat "plateau" 44 for the next period of time. Finally, thisvoltage again decays at a fairly linear rate. The lower curve 38 showsthe minimum voltage of the fourth pulse train of the four-pulsesequence. Curve 38 is referred to as the loaded voltage. The loadedvoltage is less than the open circuit voltage by about 0.6 V since theESR of the cell is about 0.3 Ω (and 0.6 V =2 A ×0.3 Ω , by Ohm's law).The ESR represents the internal resistance of the cell which causes itsloaded voltage to be less than its open circuit voltage. Curve 37showing the "background voltage," i.e., the voltage under the 17.4 KΩconstant resistive load, decreases because the open circuit voltage isdecreasing as the chemical composition of the cathode material changes.The horizontal axis of the curve shows the charge delivered by thebattery in ampere hours (Ah).

FIGS. 3 and 4 also illustrate the occurrence and effects of thephenomenon referred to as voltage delay which is present under certainpulsing conditions. This phenomenon can occur during the time in whichthe cell is in the plateau region 44 of discharge curve 37. With voltagedelay, the cell voltage at the start of a capacitor charging cycle islower than at the end. During a 10 s charging cycle, the voltage risesfrom an initial minimum voltage during the first 2 or 3 s of the pulseand then continues at the typical pulse voltage throughout the rest ofthe 10 s interval, as illustrated in FIG. 3. This phenomenon occurs onlyif the time between pulses is longer than approximately two months. Inother words, if the battery has not been pulsed during an interval of atleast two months and if the cell is in the "flat" portion of thedischarge curve, the phenomenon is observed. If the battery is pulsedmonthly or more frequently, the phenomenon is not observed. The voltagedelay can lead to a significant voltage decrease during the start ofcharging.

FIG. 5 provides, in schematic form, details of the ICD of the presentinvention. In addition to conventional ICD circuitry, such as thatillustrated in FIG. 1, there is a voltmeter 50 attached to the battery13, and a current meter 52 connected in series from the battery to theinvertor. These components of the circuit are used to monitor thevoltage and current to provide data for the maintenance procedures ofthe invention, and to provide information for scheduling of thatmaintenance. As will be recognized by one of ordinary skill in thedigital electronic arts, the implementation of the specific circuitelements of the circuit shown generally in FIG. 5 may be achieved in avariety of ways. However, due to the specific constraints imposed by ICDdesign (small volume, low mass, etc.), the actual electronicimplementation of the methods of the present invention will be somewhatlimited. By way of example, and without limitation as to the practice ofthe methods of the present invention, possible implementations ofvarious electronic circuit elements compatible with the practice of thepresent invention are provided in the accompanying figures and describedin more detail below.

Referring to FIG. 6, there is provided a timing diagram for thefunctioning of a circuit in accord with the practice of the presentinvention. The top trace 62 represents the status of the invertor switchdrive line 18 in the detail in the general schematic of FIG. 5. The nexttrace 64 represents the status of the primary current during initiationof a typical charging cycle in the method of the invention. Trace 66indicates the condition of the battery voltage during the correspondingtime period, and trace 68 indicates the specific timing of the samplingnecessary to the practice of the invention.

Referring now to FIG. 7, there is provided a level of detail to thegeneral circuit of FIG. 5 illustrating a portion of the invertorcircuitry. FIG. 7 illustrates a battery 72 and a capacitor 74 in series,along with transistor 77. Also shown is resistor 76, which functions asa sensing resistor and is characterized by a relatively low impedance,on the order of 20-50 mΩ. Contact points 80, 82 are utilized byappropriate digital or analog circuitry to establish either voltage orcurrent measurements across sensing resistor 76.

Illustrated in FIG. 8 is a digital circuit complementary to that shownin FIG. 7. Contacts 80 and 82 are common to the correspondingly numberedelements of FIG. 7. FIGS. 7 and 8, taken in combination, present a fullydigital implementation of the device and method of the presentinvention. However, as will be recognized by one of ordinary skill inthe appropriate art, the present invention can be practiced through theuse of analog circuitry as well. By way of example only, and withoutlimitation, FIG. 9 provides generally a portion of an analog circuitcapable of functioning in accord with the present invention.

Referring again to FIG. 4, the voltage threshold line 40 is shown asapproximately 2.75 volts. It is recognized that the temporary increasein ESR occurs during the plateau period 44. In order to isolate thisperiod, and to do so conservatively, the present invention provides twovoltage thresholds 40 and 42. When the voltage on the cell is below thatof threshold 40 (≈2.75 volts), and above that of voltage threshold 42(≈2.4 volts), the battery is in its plateau region, indicating thepotential need for maintenance. It should be appreciated that thevoltage data shown in FIG. 4 from which the threshold voltages for thepractice of the present invention are determined are presented forexample purposes only, and should not be taken as absolutes to beapplied in every situation. The data for FIG. 4 and the thresholdvoltages utilized in the discussion herein are representative of aparticular ICD and, as such, will need to be adjusted for varyingdevices, depending on characteristics of the circuitry and batteriesinvolved, a task that is well within the scope of one of ordinary skillin the appropriate art.

Referring now to FIG. 10, there is provided, in flow chart form, themaintenance scheduling and operation of an embodiment of the method ofthe present invention. The flow chart represents a sequence of eventsand decisions that would preferably be acted upon on at least a dailybasis. However, as would be recognized by one of skill in theappropriate art, the current state of digital circuitry design andcontrol would permit monitoring of the state of the battery according tothe present invention over a wide range of programmable time intervals.

In the first step 90 of the method of the present invention, the ICDwould determine the time elapsed since the last shock of significantenergy. As defined in this embodiment, the significant energy atdecision point 90 is a shock greater than or equal to 8 joules. As wouldbe recognized in the art, this could be more or less depending upon thetype of battery and the design of the charging circuitry. If the circuitdetermines at decision point 90 that the time since last threshold shockis less than two months, then the maintenance procedure stops. If thereis an indication that it has been greater than two months since the lastthreshold shock, then the maintenance process continues.

The next decisional point of the method is a voltage comparison 92. Theinstantaneous voltage of the battery is measured by volt meter 50 (seeFIG. 5) and compared to a pre-determined threshold voltage 40 (see FIG.4) which, by way of example, is 2.75 volts. If the voltage is greaterthan the 2.75 volts of this example, then the maintenance procedurestops, as it is known that the battery is not in its plateau region (seeFIG. 3), and is not in danger of developing an unacceptably high ESR. Ifthe voltage comparison 90 determines that the instantaneous batteryvoltage is less than the pre-determined threshold, then the maintenanceprocedure continues. The method of the present invention as representedin FIG. 10 is a somewhat simplified embodiment of the invention in thatthe decisional process employed in the method makes use only of theupper threshold voltage level 40 of FIG. 4. Thus, this embodiment doesnot make use of the additional threshold voltage 42 of FIG. 4. Theadvantage of an alternative embodiment involving checking the thresholdline 42 is that such an additional step would eliminate the use ofscarce energy for battery maintenance when the battery is very near theend of its useful life. The disadvantage of this approach is additionalcomplexity for measuring the battery voltage when the device is veryclose to the end of its life.

According to FIG. 10, the next step 94 involves the recording andstoring of V₀ which is the instantaneous unloaded voltage that wasmeasured in step 92. The next step in the method involves starting thebattery loading cycle 96 which charges the capacitors. During thisbattery loading cycle, there are two items that are checked on acontinual basis. A voltage comparison 98 is performed continuously tocompare the instantaneous output voltage of the cell to the previouslymeasured unloaded voltage (V₀), corrected for an allowed voltage drop(V_(d)). If it is determined that the instantaneous output voltage isgreater than the corrected unloaded voltage, then the battery has hadsufficient maintenance, and the maintenance procedure terminates. If thevoltage is below this level, then maintenance capacitor loading mustcontinue.

The next step of the method is a time comparison 100. The method isdesigned to initiate a charging cycle of a maximum duration of twoseconds. This is based on the assumption, derived from experimentalobservations, that charging sufficient to correct for temporary ESRconditions is generally achieved within two seconds. In comparison, thecapacitor "reforming" charge pulse is typically of ten seconds duration.Thus, if the time comparison 100 determines that the charging cycle hasnot yet been in progress for two seconds, then the method continues.However, once the two second period has passed, then the methodterminates in accordance with the above-stated empirically-basedassumption.

The pre-determined voltages, energies, and time limits that are involvedin the method of the present invention as represented in FIG. 10 are allvariables, recognizably dependant upon the size of the battery and thecircuitry. For example, as one of skill in the art would recognize, amaintenance cycle length of up to four months may be acceptable. Acharge time of one second may also be acceptable for maintenance. Thus,the variable values presented here are presented solely for the purposeof example, and are not intended in any way as a limitation on thepractice of the method of the present invention.

Referring now to FIG. 5 current meter 52 has two uses. The first is asan alternative, in another embodiment of the invention, for the voltagemeasurement of step 98 in FIG. 10. This alternative embodiment, asillustrated in FIG. 11, is possible because another indicator of therecovery of a battery is the determination of whether the battery iscapable of delivering a current in excess of a minimum threshold level.Thus, in this alternate embodiment, during the maintenance, loadingperiod, the current through meter 52 is monitored continuously at step98 of FIG. 11. When the monitoring indicates that the current is inexcess of a predetermined threshold level, then the maintenanceprocedure is terminated.

The alternative purpose of current meter 52 is for emergency backupdetermination of the maintenance state of the battery. If, for somereason, the battery maintenance was unsuccessful, then a high ESR coulddevelop. This could lead to a reduction in output current capabilitywhich could prevent the delivery of effective therapy until removal andmaintenance of the battery. If, during the capacitor charging for anormal shock, the current meter 52 detects a reduction in output currentbelow the minimum allowable level, then this would indicate that a highESR condition exists. Alternatively, the same result could also indicatethat the battery could be past the period of its useful life, althoughthere are generally available a number of means to monitor a battery forthat condition, such as early low voltage warnings and replacement. Withthis low current warning, the circuitry is then instructed to pulse thetransistor 18 at a lower rate to minimize the load on the battery 13until reconditioning was completed. This would generally require only afew seconds, after which time the circuitry could resume operation atfull current. FIG. 12 shows this embodiment. The process is started atstep 110 if a charging cycle is occurring. At step 112, a minimumcurrent determination is made. If the minimum current is not present,the charging frequency is decreased. A predetermined time period ismeasured at step 116, shown to be 2 s although other times may beindicated depending upon the circuitry and the battery. Afterward, thecharging frequency is restored to its original value. Alternatively,upon sensing a lower current or high ESR, the charging frequency couldbe reduced and then gradually ramped back up to the normal chargingfrequency.

Having thus described the preferred embodiments of the presentinvention, those of skill in the art will readily appreciate the manyother embodiments which can be employed within the scope of the claimsprovided below.

What is claimed is:
 1. An improved implantable defibrillator systemadapted to be implanted in a human patient for producing acapacitive-discharge countershock, the implantable defibrillator systembeing a self-contained device including a capacitor system, a batterysystem, a sensing system and a control system that, in response to acardiac arrhythmia in the human patient sensed by the sensing system,selectively charges the capacitor system from the battery system andthen selectively discharges the capacitor system through a plurality ofelectrodes adapted to be implanted in the human patient, the improvementcomprising:means for measuring an electrical parameter value of thebattery system; and means within the control system for controlling thecharging of the capacitor system from the battery system based upon themeasured electrical parameter value.
 2. The defibrillator system ofclaim 1, wherein the means for measuring an electrical parameter valuecomprises a voltmeter connected in a parallel configuration to thebattery system.
 3. The defibrillator system of claim 1, wherein themeans for measuring an electrical parameter value comprises a currentmeter connected in series to the battery system.
 4. A method formaintaining a battery system in a defibrillator implanted within a humanpatient comprising the device-implemented steps of:(a) periodicallymeasuring an electrical parameter value of the battery system; (b)comparing the measured electrical parameter value to a pre-determinedelectrical parameter comparison value; and (c) if the measuredelectrical parameter value is less than the pre-determined electricalparameter comparison value, initiating a battery loading maintenancecycle.
 5. A device for maintaining a battery system in a defibrillatorimplanted within a human patient to produce a capacitive-dischargecountershock, the defibrillator system being a self-contained deviceincluding a capacitor system, a battery system, a sensing system and acontrol system, the device comprising:means for periodically measuringan electrical parameter value of the battery system; means for comparingsaid measured electrical parameter value to a predetermined electricalparameter comparison value; means for initiating a battery loadingmaintenance cycle; and means for terminating said loading maintenancecycle; said defibrillator being in electrical communication with andhousing said means for periodically measuring, said means for comparingsaid measured electrical parameter, said means for initiating a batteryloading maintenance cycle and said means for terminating said loadingmaintenance cycle.
 6. The device according to claim 5 wherein said meansfor periodically measuring includes means to store data of saidpredetermined electrical parameter and a reading of state-of-charge ofsaid battery system.
 7. The device according to claim 5 wherein saidmeans for periodically measuring an electrical parameter value comprisesa voltmeter in electrical communication with said battery system.
 8. Thedevice according to claim 5 wherein said means for periodicallymeasuring an electrical parameter value includes a current meter inelectrical communication with said battery system.
 9. A power inventorymaintenance and replenishment device for a battery system wherein aloadable voltage source with a variable state-of-charge is monitored tobe maintained at a predetermined state-of-charge to reform and charge acapacitor in an implantable cardioverter defibrillator (ICD) system, theICD system being self-contained and comprising the capacitor system, thebattery system, a sensing system and a control system which incombination respond to a cardiac arrhythmia in a human patient whereinthe capacitor system is charged by the battery system to therebydischarge high voltage pulses into the human patient via electrodesimplanted in the patient comprising:means for measuring thestate-of-charge of the battery system; means for comparing said measuredstate-of-charge with said predetermined state-of-charge; means forinitiating a battery charging cycle; and means for terminating saidcharging cycle; the ICD system being in electrical communication withand housing said means for measuring, said means for comparing, saidmeans for initiating and said means for terminating said charging cycle.10. The device according to claim 9 wherein said means for measuringincludes means to store data of said predetermined electrical parameterand said state-of-charge of the battery system.
 11. The device accordingto claim 9 wherein said means for measuring includes a voltmeter inelectrical communication with said battery system.
 12. The deviceaccording to claim 9 wherein said means for measuring includes a currentmeter in electrical communication with the battery system.
 13. Anautomatic battery maintenance cycle device, triggered by astate-of-charge of a battery, for use in an implantable cardioverterdefibrillator (ICD) device wherein said ICD comprises a battery system,a capacitor system, a sensing system and a control system these systemsbeing integrated to supply a high voltage pulse to implantableelectrodes comprising:means for measuring the state-of-charge of thebattery system; means for comparing said measured state-of-charge to apredetermined electrical value; means for initiating the batterymaintenance cycle; and means for terminating said maintenance cycle;said ICD device having electrical connections to and housing saidbattery maintenance cycle device.
 14. The device according to claim 13wherein said maintenance cycle device includes a voltmeter and a currentmeter.